for x>0 and 0 elsewhere. is the scale parameter,
which is the inverse of the rate parameter .
The rate parameter is an alternative, widely used parameterization
of the exponential distribution [R168].

The exponential distribution is a continuous analogue of the
geometric distribution. It describes many common situations, such as
the size of raindrops measured over many rainstorms [R166], or the time
between page requests to Wikipedia [R167].